This invention relates to a process for the preparation of para-nitroso-diphenyl-hydroxylamines.
Para-nitroso-diphenyl-hydroxylamines may be obtained by the dimerization of nitroso benzenes in concentrated sulphuric acid (E. Bamberger et al., B. 31, page 1513 (1898)). Since the process is highly exothermic, relatively large quantities of concentrated sulfuric acid must be used to achieve adequate mixing of the components and dissipation of heat. Ice or water must be added to the mixture to isolate the reaction product and the large quantities of dilute sulphuric acid which are thus formed are neutralized and subsequently removed as effluent. The recovery of the sulphuric acid is not economically practical, partly because it is contaminated with nitrogen-containing organic material. The process is therefore not suitable for the industrial manufacture of para-nitroso-diphenyl-hydroxylamines.
In the process described in German Patent No. 2,020,043, the dimerizing rearrangement of nitroso benzenes is performed with a sulphuric acid having a concentration of at least 50%, by weight, preferably at least 75%, by weight, in the presence of an organic liquid, such an aliphatic hydrocarbon, a halogenated hydrocarbon, or an aromatic nitro compound, at temperatures of from about 5 to about 50.degree. C. Since the reaction product decomposes very rapidly in the strongly acetic solution at higher temperatures, the heat of reaction must be rapidly removed. Such removal is performed by the organic liquid, but, in such a process, the sulphuric acid must also be used in large excess, namely in up to 10 times, preferably 2.5 to 6.5 times, the molar quantity of nitroso benzene. The foregoing again gives rise to the disadvantages already described with respect to the process of Bamberger et al. Moreover, the reaction product which is obtained is of poor quality since it contains significant quantities of tarry constituents in addition to products which are sulphonated in the nucleus. When stoichiometric quantities of sulphuric acid are used, para-nitroso-diphenyl-hydroxylamine precipitates in the form of its sulphate, as a viscous, dark mass which is technically difficult to handle and also contains substantial quantities of tar, in addition to products which are sulphonated in the nucleus.
In the process described in German Patent No. 1,147,237, hydrogen fluoride is used as a dimerizing agent instead of concentrated sulphuric acid. The reaction is performed at temperatures between -20.degree. C. and 50.degree. C., optionally in the presence of an inert organic solvent (see K. Wiechert et al, Z. Chem. 15 (1955) page 21). In such a process the dimerizing agent is also used in a large excess because hydrogen fluoride serves not only as the catalyst but also as the solvent. If hydrogen fluoride is used in the stoichiometric amount, a yield of only 25% of theoretical is obtained and the product is contaminated with derivatives which are fluorinated in the nucleus. The hydrogen fluoride is distilled off under vacuum after the reaction and may be recycled. However, hydrogen fluoride has the disadvantage of having a low boiling point and a very pungent odor. Furthermore, its vapors are highly toxic to the respiratory system. Moreover, the desired product is obtained as a viscous mass which is still contaminated with about 10 to about 20% of hydrogen fluoride adhering to it. The hydrogen fluoride is virtually impossible to recover because under the necessary conditions para-nitroso-diphenyl-hydroxyl-ammonium fluorides decompose to a blackish brown mass. The wash water is therefore also contaminated with hydrogen fluoride and problems of corrosion also occur. The process, therefore, is very expensive with respect to the apparatus required, so that it is not truly suitable for the preparation of para-nitroso-diphenyl-hydroxylamines on an industrial scale.
It is also known that para-nitroso-diphenyl-hydroxyl-amine is formed together with nitrobenzene when nitrosobenzene is treated with peroxy trifluoro acetic acid (J. H. Beyer, J. Org. Chem. 24, 2038 (1959). In such a process, peroxytrifluoro acetic acid oxidizes nitrosobenzene to form nitrobenzene and concurrently catalyzes the dimerization of nitrosobenzene to para-nitroso-diphenyl-hydroxylamine. The formation of nitrosobenzene is favored at higher temperatures and the formation of para-nitroso-diphenyl-hydroxylamine at lower temperatures. In the most favorable instances, the quantity of para-nitroso-diphenyl-hydroxylamine obtained is 35% of theoretical. The process is therefore not very selective and again is unsuitable for the commercial production of para-nitroso-diphenyl-hydroxylamines.
In German Patent No. 2,703,919 (U.S. Pat. No. (4,129,740) a process is described for the preparation of para-nitroso-diphenyl-hydroxylamines by the reaction of a nitroso compound of the benzene series by itself or with another such compound, in the presence of an acid, as a catalyst. The process is characterized in that an aliphatic, cycloaliphatic or aromatic sulphonic acid having a pK.sub.a value&lt;1, perchloric acid or trifluoro acetic acid is used as the catalyst in a quantity of at least 0.5 mole per mole of nitroso compound(s), and the reaction is performed at a temperature of from about -20.degree. C. to about 60 C.